1. Field of the Invention
The present invention relates to an electronic cassette for radiographic imaging. More particularly, the present invention relates to an electronic cassette for radiographic imaging which can be charged in a wireless charging method in a manner free from problems of heat dissipation and low efficiency.
2. Description Related to the Prior Art
U.S. Pat. No. 7,866,163 (corresponding to JP-A 2006-102492) discloses an electronic cassette (electronic cassette assembly) for X-ray imaging as radiographic imaging, for use in place of a conventionally used film cassette of IP cassette (imaging plate cassette). The electronic cassette receives X-rays transmitted through an object and detects an X-ray image of information of an image of the object. The electronic cassette includes a portable housing and a radiation imaging detector or FPD device (flat panel detector). The housing is in a plate shape of a small thickness. The radiation imaging detector is contained in the housing and outputs digital data of the X-ray image.
The electronic cassette is used for the X-ray imaging system installed in an imaging room in a hospital where a body (object) of a patient is positioned in an erect posture or supine posture. Also, the electronic cassette can be used discretely in a state placed on a table where the object lies, for example hands or legs, or a state manually held by the patient. Also, the electronic cassette is adapted to a portable X-ray imaging system in a site requiring emergency medicine due to an accident, disaster or the like or in a home of a patient receiving a health care service at home.
In U.S. Pat. No. 7,866,163, carbon graphite is used for a material of the housing of the electronic cassette with a small weight to increase handlability. A wireless communication interface and a rechargeable battery are contained in the housing for a wireless communication without requiring cables of communication or a power supply. The battery contained in the housing is charged by connectively setting the electronic cassette on a battery charger.
Methods of charging the battery are a contact charging method and a non-contact charging method. In the contact charging method, a connector of the housing of the electronic cassette is physically coupled to a connector of a battery charger. The non-contact charging method is disclosed in U.S. Pat. No. 7,866,163 in which the battery in the electronic cassette is rechargeable without physical contact of a connector. A technique of wireless power transmission is utilized in the non-contact charging method for charging the battery. Examples of the wireless power transmission include an electromagnetic induction method, a resonant inductive coupling method, a capacitive coupling method (field coupling method) and a radio communication method. In the electromagnetic induction method, a current flows through a power transmission coil. A power receiving coil is positioned close to the power transmission coil with a magnetic flux, and caused by the current to induce electromotive force by electromagnetic induction for generating power. In the resonant inductive coupling method, high frequency power is input to the power transmission coil, and the power is transmitted to the power receiving coil by resonance phenomenon. In the capacitive coupling method, power is transmitted by capacitive coupling between two flat plate electrodes opposed to one another. In the radio communication method, a radio wave is transmitted to and received from antennas to acquire power.
For the non-contact charging method, the housing of the electronic cassette contains a power receiving unit, which includes the power receiving coil, a flat electrode for power receiving, and a circuit device. The circuit device has a rectifier and the like and converts receiver power of an alternate current into a direct current. A wireless power transmission apparatus transmits power to the power receiving unit, and includes a power transmission unit with the power transmission coil. For charging the battery, the electronic cassette set on the wireless power transmission apparatus. The power receiving unit receives power from the wireless power transmission apparatus, and charges the battery.
JP-A 2004-252562 discloses a data processor in which the battery is charged by the radio communication method as the non-contact charging method. The housing of the data processor contains the power receiving unit having an antenna. Also, various types of the circuit device are contained in the housing with the power receiving unit. As electromagnetic noise is created by the power receiving unit, a shield device is disposed in the data processor for shielding the circuit device from the power receiving unit in order to prevent influence of the electromagnetic noise to the circuit device.
Development of use of the non-contact charging method in the electronic cassette is a presently important concern in the field of radiography. Problems have been found in the non-contact charging method in the electronic cassette in relation to a drop of efficiency in the charging and heat dissipation of the power receiving unit. Various characteristics for improving handlability of the electronic cassette are required, such as a small weight and small thickness of the housing, and durability of the housing with resistance to load of an object. Also, the housing should have a function of shielding the electromagnetic noise (electromagnetic shielding) for preventing entry of the electromagnetic noise into the housing from external electronic devices and preventing emission of the electromagnetic noise from the circuit device of the housing to the outside. Examples of materials of the housing for satisfying those requirements are carbon materials such as carbon graphite of U.S. Pat. No. 7,866,163, and metal such as stainless steel, with a small weight, high rigidity and electrical conductivity.
An electrical conductor is used as a material of the housing of the electronic cassette. The power receiving unit is contained in the housing for the non-contact charging method. For charging, the electronic cassette is set on the wireless power transmission apparatus. The electrical conductor (part of the housing) stands located between the power receiving unit and the wireless power transmission apparatus. There is formed a transmission path of power between the wireless power transmission apparatus and the power receiving unit. The electrical conductor present in the transmission path absorbs power and causes a considerably high loss of the power. A serious problem occurs in remarkable drop in the efficiency in the charging of the rechargeable battery. Although some differences occur between the plural types of the wireless power transmission, it is likely in the worst situation that all of the transmitted power is absorbed and used up for heating of the housing without use for the proper purpose of the charging.
The power receiving unit generates heat of a considerably high amount. Circuit boards irrelevant to the power receiving unit, such as a circuit board for the radiation imaging detector, do not operate in the course of the charging. Only the power receiving unit generates heat because of its circuit board. There occurs a local increase in the temperature in an active pixel area in the radiation imaging detector positioned in alignment with the power receiving unit generating heat. A difference in the temperature occurs within the active pixel area. The characteristics of the sensitivity and dark current of the pixels are changeable with dependency on the temperature. Assuming that the electronic cassette is used for imaging with the temperature difference, changes in the density occur in an X-ray image to cause failure in the imaging. Therefore, the heat dissipation should be suitably carried out for suppressing influence of heat of the power receiving unit. The housing of the electronic cassette has the small thickness. The heat dissipation must be treated as the radiation imaging detector is vulnerable to influence of the heat of the power receiving unit.
In short, there are requirements of a small weight and small thickness in the housing of the electronic cassette. The above-mentioned problems of low efficiency in the charging and heat dissipation should be treated in consideration of reduction of the weight and thickness of the housing.
U.S. Pat. No. 7,866,163 and JP-A 2004-252562 do not disclose or suggest the above-described problems or a solution of the problems. In JP-A 2004-252562, the shield device is suggested between the power receiving unit and the circuit device contained in the housing of the data processor. There is no suggestion of the housing located in the power receiving unit between the power receiving unit and the wireless power transmission apparatus.